This invention relates to a lithographic printing plate material which utilizes the silver complex diffusion transfer process.
Lithographic printing plates consist of greasy ink receptive oleophilic image areas and ink unreceptive oil repellent non-image areas, the latter being generally water receptive hydrophilic areas. Therefore, the ordinary lithographic printing is carried out by feeding both water and ink to the printing plate surface to allow the image areas to receive preferentially a colored ink and the non-image areas preferentially water and then transferring the ink on the image areas onto a substrate such as paper.
Thus, in order to obtain a print of good quality, it is necessary that oleophilicity of the image areas and hydrophilicity of the non-image areas are both high enough so that when water and ink are applied, the image areas may receive sufficient amount of ink while the non-image areas completely repel the ink.
In the case of lithographic printing plate materials which utilize the silver complex diffusion transfer process (DTR process), especially those which have a nuclei layer on the surface of emulsion layer, silver salt grains which have formed latent image are reduced to black silver by development to form hydrophilic non-image areas while unexposed silver salt crystals become silver complex by silver salt solvents in a developer, which diffuse to the surface nuclei layer and bring about physical development due to the presence of the nuclei to form image areas mainly composed of ink receptive silver on the gelatino surface layer.
Since the silver layer precipitated on gelatin is used as ink receptive areas in the above lithographic printing plates, the image areas are insufficent in resistance against mechanical wear as compared with general lithographic printing plates (e.g., PS plate) and thus they show defects such as dropout of the image areas, gradual decrease of ink receptivity of the image areas, etc. An increase of hardness of gelatin or increase of amount of physical development nuclei in an attempt to overcome the above defects causes formation of scums and a sharp reduction of printing endurance. The mechanical strength of the image areas and scumming conflict with each other to provide a limitation of the endurance of the lithographic printing plates and development of new techniques to overcome this limitation has been demanded.
The inventors have conducted wide research on gelatins which serve as a direct support for image silver layer and as a result it has been found, surprisingly, that with decrease of content of calcium ion in gelatin, the mechanical strength of the image areas increases and furthermore the scum increases very little and printing endurance markedly increases.